The vast majority of vaccines are administered into the muscle (intramusculary, IM) or into subcutaneous fat (subcutaneously, SC) using needle and syringe. Mammalian skin is composed of two primary layers: the epidermis and the dermis. The epidermis is the outer layer of the skin and it has a thickness of 150-200 μm, while the dermis which is the inner layer has a thickness of 1.5-3.0 mm. The skin thickness varies with age, gender and anatomic site, but also racial and regional differences have been reported. Underlying the dermis is the hypodermis which is not part of the skin, but connects the skin to bone and muscle.
The standard procedure for intradermal injection of substances, i.e. injection in the dermis of the skin, is the Mantoux technique. This requires the vaccinator to judge the angle and depth of needle insertion by eye/feel, which requires special training for medical staff and may not reliably target skin as it can result in technical errors and variability of dose delivery. The Mantoux technique is nonetheless used as the route of choice for a very limited number of vaccines, such as Bacillus Calmette-Guérin for tuberculosis, and in some developing countries for rabies vaccination.
The unique immunological properties of the skin make the epidermis and dermis attractive sites for prophylactic and therapeutic vaccination. Skin dendritic cells are important professional antigen-presenting cells that have potent T-cell activating properties upon pathogenic challenge. These favourable immunological properties of the skin allow the use of reduced doses, which makes intradermal delivery of substances an attractive alternative to IM/SC delivery.
The World Health Organization has estimated that annually about 8-16 million Hepatitis B infections, 2-5 million Hepatitis C infections and 80,000-160,000 HIV infections occur because of needle stick accidents and the re-use of disposables needles and syringes. Safe ID devices that prevent needle stick accidents as well as re-use of the injecting material could provide an answer and have become a high priority for global health.